Electrochemical energy stores within the meaning of the invention can operate at any service temperature range required. However, a service temperature range of 100° C. to 500° C. is specifically preferred. This service temperature range encompasses electrochemical energy stores which operate on the basis of sodium-nickel chloride cell and sodium-sulfur cell technology. Sodium-nickel chloride cells can also be configured such that at least part of the nickel in the cell is replaced or supplemented by iron. In their anode space and cathode space respectively, energy stores of this type are provided with a corresponding anode material or cathode material which, at the service temperatures thereof, is essentially in the liquid state. Accordingly, in the case of a sodium-nickel chloride cell, the anode material is liquid sodium. In a sodium-nickel chloride cell, a cathode material is also present in the cathode space, which is at least partially in the liquid state, and comprises a salt mixture of nickel, common salt and further additives. Given the liquid inventory present in the device, the fill level of the anode material or cathode material is subject to the earth's gravitational field, and is exceptionally easy to determine as a result. The fill level or fullness level corresponds to the average fullness level of the material (anode material, cathode material) in the respective space (anode space, cathode space) during the normal use of the energy store. In conjunction with the normal use of the energy store, the fill level or fullness level changes as the service life proceeds. Where the relevant material is only intended to be partially present in the liquid phase, the definition of the fullness level will refer to the respective liquid phase of said material.
In order to obtain technically relevant voltage values from storage systems which comprise a plurality of electrically connected electrochemical energy stores of this type, a plurality of individual energy stores are customarily connected in series and combined in strings. Electrochemical storage modules include strings of this type. Accordingly, in the context of the invention, the concept of the string and the storage module are considered as equivalent concepts hereinafter.
In electrochemical storage modules of this type, in some cases, the scatter band of the capacities of individual energy stores is a substantially significant factor for the design and the functional properties of the system as a whole. Specifically, the smallest capacity of an individual energy store dictates the maximum useful capacity of the entire string of energy stores. A comparable observation can apply to the different states of charge of the individual energy stores in a string of this type. Accordingly, the state of charge of the energy store with the highest state of charge in a string of this type dictates the time at which the entire system is fully-charged, whereas the energy store with the lowest state of charge dictates the time at which the entire system is discharged. If, for example, a charging voltage continues to be applied to an individual energy store after full charging, further unwanted conversion reactions may proceed in the active interior space of the energy store, which can sometimes contribute to interference, damage, or even the destruction of the energy store. Accordingly, the further charging of a string is to be avoided, if one energy store has already achieved its full state of charge. Likewise, it is possible that the discharging of an energy store which is already fully discharged might cause impairments of this type. Accordingly, the discharging of a string is also to be avoided, if one individual energy store is already fully discharged. It is therefore a technical imperative that the energy stores in an electrochemical storage module should be charged or discharged in consideration of these defining conditions.
As already described above, electrochemical energy stores in a storage module of this type will consistently show a scatter band in respect of their capacities or states of charge. In the first instance, this scatter band is attributable to manufacturing factors, as not the entire weighed-in quantity of active material (anode material, cathode material) on the interior of the energy store contributes to the available capacity. Influences upon capacity associated with the particle size of active materials are also known. Moreover, during the operation of an energy store, distance-related electronic conduction paths may form between individual islands in the active material (percolation), resulting in a structural change, and consequently a change in the electrical parameters of the energy store. Accordingly, electrochemical reactions can only proceed in those areas of the anode or cathode which are provided with a sufficient electrical connection throughout the entire conversion time to act as a current collector for the energy store. Electrically isolated zones make no contribution, or substantially no contribution, to the conversion reaction.
Service-related changes in the individual components of the energy store may also contribute to a scatter band of the capacities or states of charge of the individual energy stores in an electrochemical storage module. It is known, for example, that thermal stresses occur during operation, which can lead to the propagation of micro-cracks in the ion-conducting separator. In the case of an energy store based upon the principle of a sodium-nickel chloride cell, this separator is a ceramic separator, comprised of Na-β-Al2O3 or Na-β″-Al2O3. Upon the occurrence of micro-cracks in this ceramic separator, for example at service temperatures, elementary sodium from the anode space can react directly with the cathode material (liquid electrolyte) in the cathode space to form elementary aluminum and common salt. As a consequence of this reaction, the electronic resistance of the ion-conducting separator might be reduced, thereby resulting in a continuous self-discharging of the energy store. If, for example, an ion-conducting separator of this type typically shows an electronic resistance of several MΩ, a damage-related reduction in the electronic resistance, for example down to 10 kΩ, at a cell voltage of 2.5V, might result in the flow of a continuous self-discharging current of 0.25 mA.
Accordingly, in the presence of an electrical series circuit of individual electrochemical energy stores, the energy store with the smallest capacity or the lowest state of charge will be the first to reach the time of complete discharging. If, at this time, the energy store continues to receive a discharge current flowing in the same direction, this might cause a polarity reversal in the already discharged energy store, generally resulting in irreversible damage, to the extent of the damage-related failure of said energy store.
Conversely, if an electrochemical storage module with series-connected electrochemical energy stores continues to be charged beyond the point in time at which the first energy store has achieved a complete state of charge, the charging voltage in the energy store which has already been charged will generally rise to unacceptably high values, whereby irreversible chemical reactions might again impair the functional components of the energy store, or damage the energy store to the extent that the latter fails.
In order to prevent potential damage to individual energy stores in an electrochemical storage module of this type during both charging and discharging, simple means are conventionally implemented in the attempt to prevent overcharging or exhaustive discharge. For example, upon the initial constitution of the electrochemical storage module, an advantageous preselection can be achieved by the targeted sorting of individual energy stores, the capacity of which does not lie within a narrow tolerance range. Likewise, in the operation of the energy store in the electrochemical storage module, only a proportion of the capacity actually available may be used. For example, only 80% of the full available capacity may be exploited, thereby reducing, however, both the flexibility and the efficiency of the system as a whole.
In part, in the conventional operation of individual electrochemical energy stores in a storage module, targeted servicing measures are undertaken in order to protect against damage associated with overcharging or excessive discharging. For example, equalization of the individual states of charge of the various energy stores can be achieved by the targeted charging or replacement of individual energy stores. However, within a given maintenance interval, such measures cannot prevent an increasing scatter in the state of charge of individual electrochemical energy stores, such that an increasing impairment in the available capacity of the system as a whole must be anticipated.